Reaction injection molding (RIM) has become an important process for the manufacture of a wide variety of moldings. The RIM process is a so-called "one-shot" process which involves the intimate mixing of a polyisocyanate component and an isocyanate-reactive component followed by the injection (generally under high pressure) of the mixture into a mold with subsequent rapid curing. The polyisocyanate component is generally a liquid isocyanate. The isocyanate-reactive component generally contains a high molecular weight isocyanate-reactive component (generally a polyol), and usually contains a chain extender or cross-linker containing amine or hydroxyl groups. See, e.g., U.S. Pat. Nos. 3,991,147, 4,764,540, 4,789,688, 4,847,307, and 4,868,224.
In general, the art has looked to various techniques for increasing the flexural modulus of a RIM part. One particularly preferred technique is the use of a reinforcing fiber mat. See, e.g., U.S. Pat. Nos. 4,792,576 and 4,917,902. Problems have been seen in using such mats in the RIM process including displacement of the mat in the mold, incomplete filling of the mold, part distortion, and surface porosity. It is believed that these problems are caused in part by the RIM reactants reaching a high viscosity in too short a time to completely impregnate the mat. Generally, the art worked toward the development of relatively low viscosity components to allow the reinforcing fiber mat to be processed in a closed mold. See, e.g., Schumacher et al, "LOW VISCOSITY REINFORCED POLYURETHANE FOAM SYSTEMS FOR INTERIOR AUTOMOTIVE PANELS",ASM International, 1987, pages 151-154. Thus, while structural foam systems (i.e., foam systems without fiber reinforcing mats) used isocyanate components having viscosities at 25.degree. C. of 300 mPa.s and isocyanate reactive components having viscosities at 25.degree. C. of 1500 mPa.s, reinforced systems used components having viscosities at 25.degree. C. of only 50 and 180 mPa.s respectively (see the Schumacher et al article). Conventional wisdom believed that since the resistance to flow through the mat is directly proportional to the viscosity of the system, the viscosity had to be kept relatively low (see, also. U.S Pat. No. 4,792,576).
It is also known that in the resin transfer molding of polyesters, variations in polyester resin viscosity do have an effect on the porosity of the molded part and that in some instances increased viscosity of the polyester resin could yield an improvement in surface porosity. However, because polyester resins are very much slower in reactivity when compared to polyurethane RIM systems, and since polyesters are not typically injected at high rates, it was not believed that a similar effect would be seen for reinforced RIM systems.